Infrared study of the phonon modes in bismuth pyrochlores

Abstract

The temperature dependence of the reflectance of the cubic bismuth pyrochlores ${\mathrm{Bi}}_{3∕2}\mathrm{Zn}{\mathrm{Ta}}_{3∕2}{\mathrm{O}}_{7}$, ${\mathrm{Bi}}_{3∕2}\mathrm{Mg}{\mathrm{Nb}}_{3∕2}{\mathrm{O}}_{7}$, ${\mathrm{Bi}}_{3∕2}\mathrm{Mg}{\mathrm{Ta}}_{3∕2}{\mathrm{O}}_{7}$, and ${\mathrm{Bi}}_{3∕2}{\mathrm{Zn}}_{0.92}{\mathrm{Nb}}_{1.5}{\mathrm{O}}_{6.92}$ (BZN) has been investigated by infrared spectroscopy. Spectra were collected from $30\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}3300\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ between 50 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the optical constants were estimated by Kramers-Kronig analysis and classical dispersion theory. In addition, BZN was studied by terahertz techniques to lower frequencies. Infrared-active phonon modes have been assigned to specific bending and stretching vibrational modes. A previously unassigned infrared mode at $\ensuremath{\sim}850\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ is discussed. A splitting of the $B\text{\ensuremath{-}}\mathrm{O}$ stretching phonon modes and $\mathrm{O}\text{\ensuremath{-}}B\text{\ensuremath{-}}\mathrm{O}$ bending modes is assigned to mixed-cation occupancy. The temperature dependence of the phonon frequencies and the damping coefficients are consistent with a decrease of both lattice constant and orientational disorder at low temperatures.